1. Field of the Invention The present invention relates to methods for extracting gallium from gallium containing solutions.
2. Discussion of the Background
Gallium is known for its low melting temperature, making it possible to produce alloys having a low melting point. Gallium has for some years been the object of considerably regained interest in particular because of the development of gallium arsenide for the electronics industry. This material is preferred to silicon as a semi-conductor in certain quite specific applications.
At the present time, a considerable part of the available gallium is obtained from Bayer liquors which are solutions of sodium aluminate resulting from the sodium hydroxide attack on bauxite. The Bayer process is a well-known process in the manufacture of aluminum trihydrate. However, despite the relatively high content of 200 to 600 mg gallium per liter of liquor in these solutions, the presence of large quantitites of aluminum makes it difficult to selectively recover gallium therefrom. The chemical properties of aluminum and gallium are very closely related. Other impurities which are soluble in a highly alkaline medium further exacerbate the problem. These are, e.g., vanadate, zincate, ferrate, molybdate.
Electrolysis over a mercury cathode has been the only process used when the quantitites of gallium to be extracted were small. But with the increasing need for this material and the problems posed by handling large quantities of mercury, there has been a movement towards liquid/liquid extraction processes, particularly those using 8-hydroxyquinoline, also known as oxine. The complex of oxine with gallium has the advantageous characteristic of being only soluble in chlorinated solvents.
Substantial progress in the development of these processes has been achieved with the appearance on the market of 8-hydroxyquinolines substituted in the seventh position, generally referred to by the term 7-(alkyl or alkenyl)-8-hydroxyquinoline, particularly those manufactured by ASHLAND OIL CO. (U.S. Pat. No. 3,637,711). This group of gallium complexing agents makes it possible to use non-chlorinated solvents.
At the same time, the problems of improving the kinetics of liquid/liquid extraction of gallium has been studied by RHONE-POULENC CO. For example, significant progress has been made by increasing the exchange surface areas between the extraction agent 7-alkenyl-8-hydroxyquinoline and the alkaline solution containing the gallium by the formation of a microemulsion (EP 0102280 and EP 0102882).
However, because considerable volumes of extraction agent and solvent are involved in this liquid/liquid extraction process and these have to be regenerated to make this extraction of gallium contained in the Bayer liquor industrially feasible, several schemes have been undertaken to extract the gallium with ion exchange resins. For instance, SUMITOMO CHEMICAL CO. has suggested extracting gallium with a resin comprising as an active group an amide-oxime function. Although apparently more appealing than liquid/liquid extraction, this process has proved to be very difficult to carry out because of the fragility of the amide-oxime groups and because the resin degrades during the successive elution cycles with an acid medium to release the gallium fixed onto the resin.
MITSUBISHI CHEMICAL INDUSTRY CO., in a Japanese Kokai patent application 60 42234/85 corresponding to Derwent ascension No. No. 85-095264, has proposed impregnating adsorbent macroporous polymer based resins with complexing agents of the 7-alkenyl-8hydroxyquinoline group, demonstrating that it was possible to fix small quantities of gallium in solution on the stationary phase. The gallium is then eluted with conventional mineral acids. However, this patent remains silent about the capacities of these resins, more precisely in terms of the gallium batches which one can hope to fix. On the other hand, all the extraction tests described have been carried out on the basis of highly diluted sodium aluminate solution, excluding any direct application of the method to Bayer industrial liquors.
Quite recently, in a communication delivered to the I.S.E.C. in Munich, in September 1986, Cote and Bauer reported having carried out studies of various adsorbent resins sold under the generic name of AMBERLITE XAD (Amberlite XAD is a trade:mark of Rohm and Haas) impregnated with 7-(5,5,7,7-tetramethyl-oct-1-en-3-yl)-8-hydroxyquinoline sold under the trade name of "KELEX"(KELEX is a trademark of Ashland Chemical Co.) The effect of various parameters such as the quantity of extraction agent or complexing agent fixed onto the resin, the concentrations of gallium, aluminum and sodium hydroxide, the chemical nature of the adsorbent resin and the effect of additives likely to activate the kinetics were studied.
From these studies, it emerges that fixing capacities in excess of 3 grams of gallium per liter of resin have been obtained with semi-polar AMBERLITE XAD7 resin having an acrylic ester skeleton. But these fixing capacities were obtained with sodium aluminate solutions having gallium concentrations which were at least five times greater than those of industrial liquors. This excludes any possibility of applying the process to industrial liquors, their concentration of gallium, in any event, not being amenable to modification.
For Bauer and Cote, the chemical nature of the adsorbent resin was far more important than its texture (active surface measured by the BET method, porosity, pore size distribution), since the authors have came to a conclusion in favor of the superiority of the adsorbent resins with an aliphatic acrylic ester chain (AMBERLITE XAD7) over adsorbent resins with a polystyrene-type aromatic nucleus (AMBERLITE XAD2), which was reported to be unsuitable.
There is therefore a need for an industrial process for recovering gallium by directly passing Bayer liquor over a resin, which requires neither prior dilution nor concentration of the liquor, while ensuring the fixing of at least 3 grams of gallium per liter of resin without rapid degradation of the stationary phase.